text: Around 4–3 Ga, anaerobic prokaryotes began developing metal and organic cofactors for light absorption. They ultimately ended up making chlorophyll from Mg(II), as is found in cyanobacteria and plants, leading to modern photosynthesis. However, chlorophyll synthesis requires numerous steps. The process starts with uroporphyrin, a primitive precursor to the porphyrin ring which may be biotic or abiotic in origin, which is then modified in cells differently to make Mg, Fe, nickel (Ni), and cobalt (Co) complexes. The centers of these rings are not selective, thus allowing the variety of metal ions to be incorporated. Mg porphyrin gives rise to chlorophyll, Fe porphyrin to heme proteins, Ni porphyrin yields factor F-430, and Co porphyrin Coenzyme B12.